1. Field of the Invention
The present invention relates generally to an electrical energy storage device, and more specifically to an electrical energy storage device for use in a solar cell.
2. Description of the Related Art
The solar energy is an advanced and sustainable development technology in all kinds of renewable energy technologies, and a solar cell is a device that converts the solar energy into electricity by the photovoltaic effect.
The most common solar cell is the silicon-based solar cell. When the sunlight hits the silicon-based solar panel, the electrons and holes can be generated and allowed to flow through the silicon-based solar panel to produce electricity. However, the photo-electric conversion efficiency of the silicon-based solar cell is determined by the crystal structure. For example, the single-crystal silicon-based solar cell has the photo-electric conversion efficiency of about 14-24%, the polycrystal silicon-based solar cell has the photo-electric conversion efficiency of about 12-20%, and the amorphous silicon-based solar cell has the minimum photo-electric conversion efficiency of 10-14%.
US. Pat. No. 2008/0141998 discloses a solar power system that can change the duty ratio D of the DC/DC converter and measure the voltage variance of the supercapacitor to enhance the efficiency of the solar power system.
Further, US. Pat. No. 2006/0197507 discloses a solar electric power supply device including an ultracapacitor assembly connected to a solar collector unit. The ultracapacitor assembly has ultracapacitors to supply the electric energy converted by the solar collector unit into a storage cell, thereby enhancing the lifetime of the storage cell.
However, the above-mentioned patents only can absorb and convert the light rays with a particular wavelength. For example, the light rays within a wavelength range of 0.2 to 2.4 micrometer can be absorbed by the silicon, but the light rays within other wavelength ranges can't be absorbed or may be converted into heat energy. As a result, the existing solar cell has low photo-electric conversion efficiency and needs to be charged during the sunlit time for providing a long charging time. Even though some solar cells are capable of receiving the light rays within a wide wavelength range, they have the disadvantages of short lifetime, unsustainable use, and high cost.